Interested in Treatment?

Sometime this year, officials in Gresham, Ore. hope to be generating all of the 15,000 kWh needed to run the city’s wastewater treatment plant every day. An expanded fats, oils and grease (FOG) program and a solar photovoltaic array are expected to save the plant about $570,000 a year on its electric bill.

Operated by Veolia Water NA, the plant serves the 119,000 people in Gresham and two nearby cities just east of Portland. The 20 mgd (design) activated sludge plant used a $40,000 grant from Oregon Economic and Community Development Department in 2010 for studying a FOG program. In the same year, the plant added a 420 kW fixed solar array.

The Oregon Association of Clean Water Agencies had included Gresham in its 2008 energy independence study. “The 2010 study really put some numbers to it,” says Paul Eckley, Wastewater Services Division manager for Gresham. “We looked at revenues through tipping fees and avoided utility costs, and did the math. It showed a payback of three or four years.”

Starting small

The Portland office of Carollo Engineers designed the FOG system, which began with a pilot program. “In 2012, we put in a 10,000 gallon tank and the pumps needed to unload tanker trucks and slowly inject FOG into our two 1-million-gallon anaerobic digesters,” says Alan Johnston, P.E., senior engineer in Wastewater Services.

To keep FOG from solidifying, the tank includes a heat exchanger that uses heat from the facility’s cogeneration system. “We didn’t have to add much — just the equipment to get FOG out of the truck and pump it to the digesters,” says Johnston. “No digester improvements were needed, and they had some excess capacity to handle the extra material.”

Haulers began delivering grease in summer 2012. “We contract with three FOG haulers who clean grease traps and grease interceptors,” says Johnston. “It’s been very successful, so we are adding another tank and associated pumps to accept more FOG.”

More grease in the digesters did not affect operations. “I think everyone was worried about that because you read a lot about how grease can cause foaming and other problems in digesters,” says Johnston. “From the day we started injecting it, it was nothing but positive.”

The 400 kW cogeneration system has operated since 2005, fulfilling about half the plant’s electricity needs. The existing gas conditioning system can handle the increased gas production. The Caterpillar 3508 lean-burn engine-generator now runs at full output around the clock and provides up to 65 percent of the plant’s power demand, saving about $250,000 a year.

Before the FOG program, the digesters produced about 180,000 cubic feet of biogas per day. Now, after completion of the first FOG program phase, they produce 280,000 cubic feet per day with addition of about 9,000 gallons of FOG daily.

Moving ahead

A second phase of the program includes a second Caterpillar 3508 engine-generator and even more biogas. That could enable the plant to produce as much energy as it consumes by the end of the year, according to Johnston. A new net metering agreement with Portland General Electric also offers financial benefit. “Before November 2012, we couldn’t send the cogenerator power onto the grid,” notes Johnston. “We had to turn down the generator whenever it started to backfeed. Now we can run it at full power 24 hours a day.”

At present, there is no plan to add gas storage. The digesters’ floating covers provide 300,000 cubic feet of storage.

While providing a new source of biogas-fueled power, FOG has also created a new revenue stream. “We’re getting about $22,000 a month in FOG tipping fees,” says Johnston. “The haulers pay us 8 cents a gallon. Without those revenues, the million dollars we’re spending on this wouldn’t make sense. When we’re all done, we think we’ll be at about 12,000 gallons a day and about $300,000 dollars a year in FOG revenue.”

If that materializes, the net savings and revenue increase will total more than $800,000 a year on an annual treatment plant budget of $4.5 million. “It’s tough to stabilize rates,” says Eckley. “This will keep rates from increasing as fast.”

Tapping opportunities

Since 2005, the Gresham plant used some $3 million in grants and other funding assistance, including money from the Oregon Energy Trust and Oregon Business Energy Tax Credits, accounting for about 30 percent of the costs of its various energy projects.

For the first-phase FOG equipment, the Energy Trust provided $40,000, and the Oregon Department of Energy (ODOE) provided $183,000 toward a construction cost of $760,000. The Energy Trust is funding another $40,000, and ODOE is providing $1 million toward the $2.9 million cost of the second phase. The Energy Trust also provided $500,000 to install the solar project, which provides about 8 percent of the plants electricity.

In addition to energy production, the plant has completed energy conservation projects that upgrade the digester mixing process by replacing three 40 hp gas compressors with a pair of 5 hp linear motion mixers. In addition, two centrifugal blowers for the aeration process were replaced with APG-Neuros turbo-blowers, and diffusers were switched to fine-bubble units.

Eckley notes that the city has the lowest home sewer rates in the area: “The residential rate is $26.30 a month. These kinds of projects help us maintain that.”